FIG. 2 shows a conventional example of this type of motor-driven valve. A motor-driven valve 1′ illustrated as a conventional example basically includes a valve body 10 having a valve chamber 14 and a valve seat 15 formed inside thereof, a circular cylindrical shaped can 30 having a ceiling part that is fixed to the valve body 10 via a base plate 31, a stepping motor 63 including a stator 40 attached outside of the can 30 and a rotor 50 attached inside of the can 30, a planetary gear reduction mechanism 60 to decrease the rotation speed of the rotor 50, a valve element 21 to control the amount of fluid passing while moving close to or away from the valve seat 15, and a screw drive member 22 to convert the rotary motion of an output gear 57 of the planetary gear reduction mechanism 60 into the linear motion via a thread feeding mechanism 27 to drive the valve element 21.
The valve body 10 includes a valve port 16 defined so as to communicate with the valve chamber 14, and a pipe 11 is connected to a part thereof on the valve port 16 side, and a pipe 12 is connected so as to communicate with an opening formed at the lateral face of the valve chamber 14. A threaded bearing member 13 having an internal thread 13a formed at a lower half part thereof on the center is inserted and fitted to the upper part of the valve chamber 14 of the valve body 10, and is fixed to the valve body 10 by caulking (caulking part 17).
The stator 40 fitted to the outer circumference of the can 30 includes a yoke 41, a bobbin 42, a coil 43, a resin mold 44 and the like, and the rotor 50 rotatably (without moving vertically) supported inside of the can 30 is made up of a circular cylindrical rotor member 51 made of a magnetic material and a sun gear member 52 made of a resin material that are coupled integrally. A shaft 62 is inserted at a center part of the sun gear member 52, and an upper part of the shaft 62 is supported by a supporting member 61 disposed inside of the top part of the can 30.
The sun gear member 52 has a sun gear 53 that meshes with a plurality of planetary gears 55 that is rotatably supported by shafts 56 provided at a carrier 54 placed on the bottom face of the output gear 57. The upper half of each planetary gear 55 meshes with an annular ring gear (fixed internal gear) 58 attached by caulking to the upper part of a circular cylindrical member 18 fixed to the upper part of the valve body 10, and the lower half of the planetary gear 55 meshes with an internal gear 57a of the annular output gear 57. The ring gear 58 and the internal gear 57a of the output gear 57 have slightly different numbers of teeth, whereby the number of rotations of the sun gear 53 is transmitted to the output gear 57 at a large reduction gear ratio (such a structure of the gears is called a mechanical paradox planetary gear reduction mechanism 60).
The output gear 57 slidably comes into contact with the upper face of the threaded bearing member 13, and the upper part of a stepped circular cylindrical output shaft 59 is press-fitted to the center of the bottom part of the output gear 57, whereas the lower part of the output shaft 59 is rotatably inserted into an insertion hole 13b formed at the upper half of the center part of the threaded hearing member 13. Then, the lower part of the shaft 62 is fitted to the upper part of the output shaft 59.
The internal thread 13a of the threaded bearing member 13 threadably engages with an external thread 22a of the screw drive member 22 (this may be called a driver as well) making up a valve shaft 20, and the screw drive member 22 converts the rotary motion of the output gear 57 (i.e., the rotor 50) into the linear motion in the direction of axis line O (vertically ascending/descending direction) through the thread feeding mechanism 27 including the external thread 22a and the internal thread 13a. Herein, the output gear 57 rotates at a fixed position in the direction of axis line O without moving vertically, and a plate-like portion 22b having a shape like a flat-bladed driver provided at the upper end part of the screw drive member 22 is inserted into a slit-like fitting groove 59b provided at the lower end part of the output shaft 59 coupled with the output gear 57, whereby the rotary motion of the output gear 57 is transmitted to the screw drive member 22. The plate-like portion 22b of the screw drive member 22 slides in the direction of axis line O in the fitting groove 59b of the output shaft 59, whereby as the output gear 57 (rotor 50) rotates, the screw drive member 22 linearly moves in the direction of axis line O due to the thread feeding mechanism 27, although the output gear 57 does not move in the direction of rotation axis. The linear motion of the screw drive member 22 is transmitted to a shaft-like valve element 21 via a ball-shaped fitting 25 including a ball 23 and a ball receiving seat 24, so that the valve element 21 is guided by a stepped circular cylindrical spring case 19 internally fixed to the valve body 10 to move in the direction of axis line O. The space between the spring case 19 and the valve element 21 stores a compression coil spring 26 in a compressed manner so as to always bias the valve element 21 in the valve opening direction.
With this configuration, the flow passage area (valve opening degree) between the valve element 21 and the valve seat 15 changes, whereby the flow rate of refrigerant passing through the valve port 16 can be controlled.
Meanwhile, in order to allow for smooth linear motion of this screw drive member, a certain degree of gap is required at an engagement part between the plate-like portion of the screw drive member and the fitting groove of the output shaft as described above. The rotor (the output shaft coupled with the output gear) rotates in two directions to control the position of the valve element in the direction of axis line via the screw drive member, and hysteresis inevitably occurs at the rotation of the output shaft and the rotation of the screw drive member as shown in FIG. 3 due to the gap as described above when the rotation direction changes.
JP 2015-014306A and JP 2012-197849A propose prior art to solve such a problem.
According to the prior art described in JP2015-014306A, the overlap width between the lateral face of the fitting groove and the lateral face of the plate-like portion that mutually overlap when viewed from the axis direction (vertically ascending/descending direction) is set larger than the inner diameter of the internal thread of the threaded bearing member.
According to the prior art described in JP 2012-197849A, the output gear can ascend/descend in a planetary gear reduction mechanism, and the output shaft and the screw drive member are shaped integrally to form the output gear and the screw drive member integrally.
The prior art described in JP 2015-014306A, however, is configured so that rotary motion of the output gear (rotor) is transmitted to the screw drive member by engagement (mesh) of the fitting groove of the output shaft and the plate-like portion of the screw drive member, and so this cannot solve the problem on hysteresis as stated above sufficiently.
The prior art described in JP 2012-197849A has the possibility of increasing the overall height to keep the strength of the output gear when the gear descends in the planetary gear reduction mechanism, and it has to additionally include a disc spring, for example, to control the position of the rotor in the axis direction, which means a complicated structure.